<p>Sandy pebble soil presents issues such as loose surrounding rock and collapse during tunnel excavation due to its sensitivity to moisture content and compactness. This study combines indoor large-scale triaxial tests and numerical triaxial tests using particle discrete elements to calibrate the microstructure parameters of sandy pebble soil under different conditions. The relationship between shear strength parameters and moisture content and compactness is established. The barn effect principle is introduced to develop a method for calculating tunnel surrounding rock load, fully considering the influence of compactness and moisture content. Key findings include: (1) increased stress peak with higher compactness and lower moisture content, and increased internal friction angle with decreasing moisture content or increasing compactness; (2) identification of critical microstructure parameters affecting strength properties, such as particle contact modulus, friction coefficient, and porosity; (3) validation of the Janssen model for sandy pebble soil layer load calculation; (4) derivation of a formula for calculating the vertical load of sandy pebble soil tunnel surrounding rock, revealing the influence of compactness and moisture content on lateral pressure.</p>